Taking X-ray images of a patient at different wavelengths of light is very useful since different body features are highlighted at different wavelengths of light.
The process of compiling radiographic images of a patient at different wavelengths of light is commonly referred to as “polychromic” X-ray imaging.
Existing systems for taking polychromic X-ray images typically involve first imaging the patient with a first X-ray beam at a first wavelength of light, and then first imaging the patient with a second X-ray beam at a second wavelength of light.
A disadvantage of this approach is that the beam emitter used must be configured to selectively emit an X-ray beam at (at least) two different wavelengths. Moreover, another disadvantage is that the patient is exposed to two different X-rays, one after another. For health reasons, it is always desirable to limit the number of X-ray exposures to the patient. An additional disadvantage is that the two X-rays are recorded at different times (during which time the patient may have moved).
In other existing systems, at least two stacked X-ray-sensitive detectors are used. The first detector is preferentially sensitive to low-energy X-rays, is partially transparent to higher-energy X-rays, and is located between the object being imaged and the second detector. The second detector is preferentially sensitive to high-energy X-rays. Both layers are exposed using a single exposure from the X-ray beam. This so-called “single-exposure, dual-detector” technique overcomes the motion mis-registration problem of the dual-exposure technique.
For example, an X-ray-sensitive detector may be used that includes an X-ray-sensitive scintillator layer to convert each incident X-ray photon into numerous optical (visible) photons. The X-ray sensitive scintillator layer is coupled to a light-sensitive detector in order to make a recording (image) of the incident X-ray photons. For a “single-exposure, dual-detector” technique, at least two light-sensitive detectors are therefore required. This represents a burden of complexity and expense.
U.S. Pat. No. 4,029,963 to Alvarez discloses a method of recording X-ray images with energy sensitivity wherein two stacked X-ray-sensitive layers are constructed of distinct scintillator materials such that the first layer (preferentially sensitive to low X-ray energies) generates optical photons of a first color, and the second layer (preferentially sensitive to high X-ray energies) generates optical photons of a second, substantially different color. These two scintillator layers are optically coupled to a single color-sensitive photographic film located between the two scintillator layers with the result that the optical signals from the two X-ray sensitive layers are recorded as different colors on the single color film. This method has the disadvantages that the film must be developed, and that if further processing of the images is desired, for example to produce derived images of either bone or soft tissue, the image on the film must be digitized.
U.S. Pat. Nos. 5,216,252; and 5,451,793 and the system described in Radiology (June 1992; 183(3):863-70) by Boone describe a binary screen detector system for single-pulse dual-energy radiography, and a method for single-exposure (called by them “single-pulse”) energy-sensitive X-ray imaging involving a single X-ray-sensitive layer, wherein the X-ray-sensitive layer (called by them a “binary screen”) comprises a mixture of two scintillator materials. As in the above Alvarez system, described above, the scintillator materials are chosen such that one scintillator material is preferentially sensitive to low-energy X-rays and generates optical photons of a first color (first wavelength), and another scintillator material is preferentially sensitive to high-energy photons and generates optical photons of a second color (second wavelength.) In this system, the two scintillator materials are mixed to form a single scintillator layer. The single layer is simultaneously viewed by two different optical cameras, the first camera having an optical filter that makes it sensitive to the optical emission of the low-energy scintillator, and the second having an optical filter that makes it sensitive to the optical emission of the high-energy scintillator. This system has the advantages that only a single exposure (pulse) is required, and that the cameras can be electronic thereby providing immediately images that can be further processed by computer.
Unfortunately, a disadvantage of this system is that, due to the finite size (small numerical aperture) of the feasible coupling lenses required to form images of the optical emissions of the X-ray sensitive layer on the respective optical detectors, a large fraction of the optical photons generated by X-ray-sensitive layer are not received by the optical detectors and are therefore lost. As stated by the inventors, this results in an optical “quantum sink” with the effect that the system is not as efficient in recording incident X-rays as desired, that is, that the system has a low detective quantum efficiency (DQE.)
What is instead desired is a simple system in which a polychromic X-ray image can be generated from a single X-ray beam imaging the patient. It is also desirable that such system does not require two separate camera systems. It is also desirable that such image be acquired in a digital format.